QUESTION: I recently had an argument regarding tread depth with the manager of a well-known used car dealership.

I have looked at Ministry of Transportation regulations on the Internet but can only find information for farm vehicles. The question: Is the legal tire tread depth of 3/32-inch measured from the bottom of the groove or the wear bar?

Karl

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ANSWER: Actually, the standard is 2/32-inch for cars and light trucks.

The Canadian Rubber Association, which represents the tire manufacturers, says that is the federal standard, enforced by the various law enforcement organizations across the country when laying charges under the authority of the Motor Vehicle Safety Act.

The association says the tread is measured from the bottom of the groove, not the top of the wear indicator bar. That is the purpose of the wear bars, to tell you when the tire has reached minimum legal tread depth.

Once a tire reaches that point of wear, it has lost much of its traction abilities and should be replaced.

With only this much tread left, the tire is unable to disperse water or cope with the heat generated by cornering on dry roads.

If the police, in investigating a crash, determine that a loss of control, or failure to stop or turn in sufficient space, contributed to a crash or death or injury and that one or more tires on a vehicle involved had less than the legal minimum tread depth, they can lay charges.

The most accurate measure of tread depth comes from a tread depth indicator used by tire shops and available from Canadian Tire or any auto supply store.

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Failing that, the easiest way to measure tread depth is to use a coin.

Grab a dime and you're ready for the Bluenose test. Insert the coin into a groove you wish to measure. If you can see the top of the sails of the famous East Coast racing schooner, it is time to replace the tire.

Real fuel economy

QUESTION: I recently bought a 2010 Subaru Outback PZEV. I made the choice largely because of the reduced emissions and the relatively good fuel economy (listed as 6.9 litres/100 km highway and 9.5 city).

But after only 500 kilometres of driving (90 per cent city), the fuel economy gauge indicates I'm using 16 L/100 km.

Is it normal, as my dealer claims, for a brand new car to consume more fuel as the engine "settles in"? Is it possible the nature of my driving is a factor (several short, three- to five-km trips a day in the city)?

Stan

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ANSWER: I suspect the problem is all of the above.

The federal EnerGuide ratings are a "best-case" scenario, a calculation based on a combination of separate tests conducted on a rolling-road dynamometer in a temperature and humidity-controlled laboratory.

The city cycle consists of a 29-km drive in simulated stop-and-go traffic beginning with a cold start - although it is hardly that at 20 degrees C. The average speed is less than 60 km/h and there are 18 stops. Throughout the test, only mild or partial throttle opening is employed.

The relatively warm start and minimal acceleration alone make the numbers less relevant to Canadian conditions.

The Americans added three new test cycles last year to try to come closer to real-world numbers - one for higher speeds, one involving air conditioning use in hot conditions and a third for cold conditions.

The results, combined with those from the two earlier tests, are more credible. City fuel use numbers dropped 12 per cent on average and as much as 30 per cent.

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Just to show the difference in U.S. and Canadian ratings, the 9.5 litres/100 km city rating for your car converts to 24.8 miles per U.S. gallon. The U.S. rating for that same vehicle is 19 miles per gallon.

Going the opposite way, converting the American rating to metric results would give you 12.4 litres/100 city - closer to your actual experience.

I'd chalk up the majority of the remaining difference to your short trips where the engine does not get an opportunity to come up to full operating temperatures. Your driving style may account for some as well.

And, yes, the numbers should improve slightly as the car gets some mileage under its belt.

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